High voltage generator and transmission transformers form an integral part of any electrical power generation, distribution and transmission system. Other transformers, such as rectifier transformers are also used in industrial processes, such as smelting and electro-deposition processes. Also, current transformers (CTs) are used for protection and metering of electricity distribution systems.
The most important part of the insulation for oil filled transformers comprises paper which is wound around the copper windings and the oil itself. There are spacers, washers, seals, lead through plates, taps and bushings, which are also part of the insulation system within the transformer. In order to enhance the insulation and stability, the paper is permeated with a dielectric, typically mineral oil or silicone oil, which fills the transformer. This insulating oil also serves as a coolant by distributing heat by convection or forced flow and also quenches discharges.
Other types of transformers include solid filled transformers which use polymeric dielectrics. This epoxy thermoset is vacuum back-filled into the transformer. There are also gas-filled transformers for example, those used in underground mines. Such transformers are usually filled with gases such as argon or sulfur hexafluoride for safety. There are also some low voltage air filled transformers.
The operating lifetime of a high voltage transformer can be greater than 35 years. The lifetime depends on the loading, design, quality of manufacture, and the materials and maintenance routines. During its lifetime, the transformer insulation can degrade, the rate of degradation being dependent upon the workload and the internal operating environment of the transformer, such as temperature, moisture content, pH and the like. Any degradation of the insulation, such as electronic and ionic plasma erosion of solid insulation surrounding an air bubble occluded due to faulty manufacture, can result in increasing levels of partial discharge within the solid thermoset filled transformer. Occurrence of partial discharges in mineral oil also leads to evolution of gases such as hydrogen and acetylene within the transformer. Such increased partial discharge leads to further degradation of the insulation which in turn leads to increasable levels of partial discharge. Continued degradation of the insulation can result in severe discharges, short-circuit faults or a catastrophic failure due to an explosion of the gases, for example, hydrogen, acetylene and ethylene, produced as chemical by-products of the degradation process. Such failure can result in reduction or loss of supply (outage) to the power system, incur considerable expense for the replacement or repair of the transformer and also present a serious risk to nearby personnel and the environment.
Partial discharge in transformers can also occur due to faulty manufacture and or mechanical or electrical fatigue. For example the movement, creep and stress relaxation of metallic components, such as fastenings or foreign metallic bodies within the transformer, provide an opportunity for discharges to occur even when there has been no or little degradation of the insulation.
Partial discharge in transformers can also arise due to windings becoming loose within the transformer. Wear and tear suffered by the tap connectors and backlash in the tap changer can also cause partial discharges and arcing. Faults in the bushings can also result in partial discharges.
It is known that a partial discharge can produce signals at different locations within a large transformer including a discharge current in neutral caused by imbalance, a displacement current through the capacitive tapping of a bushing, a radiated radio frequency (RF) pulse or wave and a radiated ultrasonic (US) pulse or wave.
The magnitude of partial discharge within a transformer provides one means of determining the integrity of the transformer's insulation. For example, a detected partial discharge having a magnitude of 50 pC would normally be ignored at normal voltage operations, a reading of 500 pC would be viewed with some concern, whilst a reading of 500 pC would be considered potentially dangerous. Just as important is the frequency of occurrence or activity of the discharges. For example, 200 pC to 500 pC occurring frequently can do more damage than 1000 pC occurring infrequently.
Power authorities typically test transformers by sampling the mineral oil within the transformer about once a year to analyse and determine the oil's dissolved gas concentration (DGA) and dielectric loss angle (DLA). If high gas readings are obtained the frequency of sampling may be increased to monthly and even weekly. However, there is always some delay between the sampling and the analysis in the laboratory. There is also delay in carrying out diagnosis based on this analysis. Rapid deterioration of insulation may not be detected early enough and transformers have failed catastrophically even when dissolved gas analysis (DGA) sampling has been carried out Since it is known that partial discharges of high magnitude and high repetition rate develop shortly before a major failure, continuous monitoring of electric equipment while it is kept on-line to provide early warning is very desirable.
Partial discharge can also be measured using instruments such as Robinson, Haefly or Tettex partial discharge detectors by coupling to the lower part of the bushing on the transformer or to the windings using capacitor dividers and a toroid system. Such detectors detect high frequency electrical signals only. These instruments are normally used in a test bay during high voltage proving tests for a new or re-wound transformer. These measurements can, however, normally not be undertaken in a substation location due to the high level of electrical noise interference. Making reliable readings with these instruments also requires considerable skill.
One device for detecting the occurrence of a single partial discharge event in a transformer is described in International Application No PCT/AU94/00263 (WO 94/28566). This device comprised an ultrasonic transducer and a radio frequency antenna that were mounted through the transformer wall or roof and adapted respectively to detect the ultrasonic and radio frequency pulses generated by a partial discharge. If a radio frequency signal was detected within a pre-set time period before detection of an ultrasonic signal, a partial discharge was assumed to have occurred. While able to detect such signals, one problem with the device described in WO 94/28566 was that electrical noise within the transformer would generate randomly occurring radio signals that lead to the triggering of false alarms on occurrences of partial discharge. Shutting down a transformer based on a false alarm is clearly undesirable and costly.
An improved device for the detection of partial discharge is described in International Patent Application No PCT/AU00/01028 entitled “Partial discharge monitoring system for transformers” which was developed by one of the co-inventors of the present application. This improved device uses signal processing techniques to discriminate detection of signals indicative of partial discharge from other signals generated due to the noisy electromagnetic environment normally present in on-line high voltage electrical equipment, such as transformers.
Both of the devices described above-rely on fixed transducer heads being mounted to the electrical equipment that is being monitored by the device. Such an arrangement is practical and cost-effective where the electrical equipment being tested is a large and expensive high voltage transmission and generator transformer. In the case of relatively cheaper small transformers or where it is desired to institute screening by monitoring for partial discharge of transformers already in use, the costs of mounting a fixed head transducer arrangement and monitoring the transformer may be uneconomic.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.